docs: def get_step_f(step_f, lR2, lS2):
      """Update the stepsize of given the primal and dual errors.
  
      See Boyd (2011), section 3.4.1
      """
      mu, tau = 10, 2
      if lR2 > mu*lS2:
          return step_f * tau
      elif lS2 > mu*lR2:
          return step_f / tau
      return step_f